itunes dont sync downloaded apps to my iphone!

Hello

i just authorised my pc and downloaded some apps by itunes,

in apps tab i click on <install> (across the new apps icons) and it turn to <will install>

BUT the problem is:

when i click on sync it first checks and creates backup and in 3rd step it says checking app to intall BUT it suddenly stops...

no error or anything just suddenly stop sync....

what should i do to make it work?

Thanks